JP 2007-064268 A describes an exemplary automatic transmission mounted on a vehicle. In this automatic transmission, an engine rotation-driven oil pump supplies hydraulic pressure to a hydraulic control circuit. A hydraulic valve and a manual valve are provided in the hydraulic control circuit to switch a hydraulic circuit and to control a hydraulic pressure. Here, the hydraulic pressure is applied to friction engagement elements (e.g., a brake or a clutch) of a transmission mechanism. As a result, gear shifting, or range shifting, is performed.
This kind of automatic transmission includes a hydraulic control system. Specifically, when the vehicle is stopped while the transmission is in the D range (i.e., the drive range), the transmission is maintained in the first gear position (i.e., the gear position used for the vehicle to accelerate from standstill). To do so, a hydraulic control valve is controlled to maintain the hydraulic pressure applied on friction engagement elements, which are engaged during the first gear position of the transmission, at an engagement hydraulic pressure. Accordingly, the friction engagement elements are maintained in an engaged state.
However, when the vehicle is stopped while the transmission is in the D range, if the hydraulic control valve is controlled to maintain the hydraulic pressure applied on the friction engagement elements at the engagement hydraulic pressure such that the friction engagement elements are maintained in an engaged state for a long period of time, there is a concern that fuel consumption may worsen due to a large energy loss.
In this regard, when the vehicle is stopped while the transmission is in the D range, it is known that an N-control process may be performed to reduce the energy loss. Specifically, the N-control process controls the hydraulic control valve to reduce the hydraulic pressure applied to the friction engagement elements which are normally engaged during the first gear position of the transmission. As a result, the friction engagement elements become disengaged (in other words, the transmission switches to a neutral state).